Christopher C. Barney

Effect of an angiotensin converting enzyme inhibitor (SQ 14,225) on β-adrenergic and angiotensin-induced thirsts☆

The effect of acute administration of SQ 14,225, a new angiotensin converting enzyme inhibitor, on the drinking response of female rats administered either isoprenaline, angiotensin I, or angiotensin II was studied during 2 h after treatment. Administration of isoprenaline (25 micrograms/kg body wt) was accompanied by a significant increase in water intake when compared with saline-treated controls. Acute administration of a constant dose of isoprenaline (25 micrograms/kg body wt) and increasing doses of SQ 14,225 (5--50 mg/kg) was accompanied by a dose-related, linear decrease in water intake. Acute administration of either angiotensin I or angiotensin II (200 micrograms/kg body wt) was accompanied by a significant increase in water intake. The dipsogenic response to angiotensin II was not affected by acute administration of 35 mg SQ 14,225/kg body wt. However, at the same dose of SQ 14,225, angiotensin I-induced thirst was attenuated. Since isoprenaline-induced and angiotensin I-induced, but not angiotensin II-induced, thirsts are blocked by SQ 14,225, the results suggest that isoprenaline-induced thirst is mediated by way of the renin--angiotensin system.

On the mechanism of serotonin-induced dipsogenesis in the rat ☆

Subcutaneous administration of l-5-hydroxytryptophan (5-HTP), the precursor of serotonin, to female rats induces copious drinking accompanied by activation of the renin-angiotensin system. Neither a reduction in blood pressure nor body temperature accompanied administration of 5-HTP. The objective of the present study was to determine whether serotonin-induced dipsogenesis, like that of 5-HTP, is mediated via the renin-angiotensin system. Serotonin (2 mg/kg, SC)-induced drinking was inhibited by the dopaminergic antagonist, haloperidol (150 micrograms/kg, IP), which also inhibits angiotensin II-induced drinking. Both captopril (35 mg/kg, IP), an angiotensin converting enzyme inhibitor, and propranolol (6 mg/kg, IP), a beta-adrenergic antagonist, blocked serotonin-induced dipsogenesis. The alpha 2-adrenergic agonist, clonidine (6.25 micrograms/kg, SC), which suppresses renin release from the kidney, attenuated serotonin-induced water intake. The dipsogenic responses to submaximal concentrations of both serotonin (1 mg/kg, SC) and isoproterenol (8 micrograms/kg, SC) were additive rather than interactive suggesting that similar pathways mediate both responses. The serotonergic receptor antagonist, methysergide (3 mg/kg, IP), inhibited serotonin-induced drinking but had no effect on isoproterenol (25 micrograms/kg, SC)-induced dipsogenesis. However, neither serotonin (2 mg/kg, SC) nor isoproterenol (25 micrograms/kg, SC)-induced drinking was inhibited by cinanserin (25 micrograms/kg, IP). These data indicate that serotonin induces drinking in rats via the renin-angiotensin system. However, the results of the studies using methysergide suggest that serotonin appears to act at a point prior to activation of beta-adrenoceptors in the pathway leading to release of renin from the kidneys.

Peripheral conversion of L-5-Hydroxytryptophan to serotonin induces drinking in rats ☆

Female rats administered serotonin (0.25 to 4.0 mg/kg, s.c.) showed a dose-dependent increase in water intake. The dipsogenic response was nearly maximal when 2 mg/lg was administered s.c. and plateaued by 2 hr after treatment. l-5-Hydroxytryptophan (5-HTP), the precursor of serotonin, is also a potent dipsogen which induces drinking by way of the renin-angiotensin system. The possibility that the dipsogenic activity of 5-HTP is dependent on decarboxylation to serotonin was the objective of these studies. Either benserazide (30 mg/kg. s.c.), a central and peripheral decarboxylase inhibitor, or carbidopa (6.5 mg/kg, s.c.), a peripheral decarboxylase inhibitor, was administered 15 min prior to the dipsogen. Both decarboxylase inhibitors attenuated the dipsogenic response to 5-HTP (25 mg/kg, s.c.) but not to serotonin (2 mg/kg, s.c.). The peripheral serotonergic receptor antagonist, methysergide (3 mg/kg, i.p.), blocked the dipsogenic responses to both 5-HTP (25 mg/kg, s.c.) and serotonin (2 mg/kg, s.c.). There was no interaction between 5-HTP (18 mg/kg, s.c.) and serotonin (1 mg/kg, s.c.) when administered simultaneously with respect to their dipsogenic effects. Thus, the drinking response accompanying administration of 5-HTP occurs following peripheral conversion to serotonin which, in turn, activates peripheral serotonergic receptors. The mechanisms(s) by which activation of peripheral serotonergic receptors increases water intake is not known, but appears to involve release of renin from the kidney.

Effects of serotonin and L-5-hydroxytryptophan on plasma renin activity in rats ☆

The effects of dipsogenic doses of l-5-hydroxytryptophan (5-HTP) and serotonin on plasma renin activity (PRA), blood pressure, and body temperature were determined in unanesthetized female rats. Both serotonin (2 mg/kg, s.c.) and 5-HTP (25 mg/kg, s.c.) induced six-fold increases in PRA measured 1 hr after drug administration. The central and peripheral decarboxylase inhibitor, benserazide (30 mg/kg, s.c.), as well as the peripheral decarboxylase inhibitor, carbidopa (6.5 mg/kg s.c.), prevented the increase in PRA associated with administration of 5-HTP. This suggests that 5-HTP must be converted to serotonin peripherally to increase PRA. At the doses used, serotonin decreased mean blood pressure and colonic temperature of unanesthetized rats while 5-HTP was without effect. The increase in PRA induced by 5-HTP does not appear, therefore, to be a response to either hypotension or a decrease in colonic temperature. Since 5-HTP must be converted to serotonin to initiate both a drinking response and an increase in PRA, the results suggest that the decrease in blood pressure and colonic temperature following administration of serotonin may not be important in induction of the drinking response and the increase in PRA. The mechanism by which activation of the renin-angiotensin system occurs following peripheral administration of either 5-HTP or serotonin remains for further study.

Effect of Chronic Treatment with Desoxycorticosterone on the Dipsogenic Response of Rats to Isoproterenol and Angiotensin

Administration of desoxycorticosterone (DOC) at a dose of 0.5 mg/kg/day to male rats for 6 weeks attenuated significantly the drinking response to acute subcutaneous administration of 5, 10 and 20 microgram isoproterenol/kg body weight. Chronic administration of DOC did not prevent the drinking response to acute intraperitoneal administration of angiotensin II (200 microgram/kg body weight). In contrast, the drinking response was enhanced compared with that of control rats given angiotensin II acutely. While attenuation of the drinking response to isoproterenol in DOC-treated rats may be attributed to depletion of renin from their kidneys, the mechanisms responsible for the enhanced drinking response to angiotensin II are not clearly understood.

Alteration of peripheral β-adrenergic responsiveness in fasted rats

Total food deprivation for 72 hrs (3 day fast) in female rats resulted in a reduction in serum thyroid hormones as well as a reduced peripheral beta-adrenergic responsiveness to isoproterenol. Food deprivation for 48 or 72 hrs significantly decreased both serum T3 and T4 values as compared to non-fasted controls. There were no significant differences in either T3 or T4 levels as a result of a 24 hr fast. Rats deprived of food for 72 hr had significantly smaller increases in oxygen consumption, colonic and tail skin temperatures following administration of isoproterenol (100 micrograms/kg b.w., s.c.) when compared to non-fasted control rats. Arterial blood pressure and heart rates were measured in unrestrained, unanesthetized, chronically cannulated rats. Food deprivation for 72 hrs significantly attenuated the decrease in blood pressure and the increase in heart rate associated with administration of isoproterenol (10 micrograms/kg b.w., s.c.). Possible mechanisms for the reduced beta-adrenergic responsiveness associated fasting are discussed.

Effect of water temperature on isoproterenol-induced water intake.

Summary When female rats were administered the β-adrenergic agonist, isoproterenol (25 μg/kg body weight, s.c.), and allowed access to water at 6.0, 10.5, 25.0, 29.6, 35.8 and 39.3°, to drink, increasing amounts of water were ingested during the 1 hr test period as water temperature increased up to 29.6°. Water intake decreased when water at temperatures higher than this was offered. A similar increase in water intake with increasing water temperature was also observed in untreated control rats. Thus, the effect of water temperature on water intake was not a function of the thirst induced by administration of isoproterenol but is a characteristic of normally hydrated rats.

Effect of cycloheximide on temperature regulation in rats.

The effect of cycloheximide, an inhibitor of protein synthesis, on temperature regulation in afebrile rats was studied to determine whether its reported antipyretic effect might be attributable to a non-specific antimetabolic effect. Within 60 min after administration of cycloheximide (5 mg/kg IP) to female rats at ambient temperatures of 34, 25 and 15 degrees C, a significant decrease in colonic temperature was observed as compared with control rats administered saline IP. Measurements of rate of oxygen consumption showed that cycloheximide (5 mg/kg IP) significantly depressed the ability of rats to increase their heat production during a cold stress (15 degrees C). In the cold the rate of oxygen consumption increased 8.7 +/- 0.8 ml/min/kg0.75 in the control rats but only 1.8 +/- 0.9 ml/min/kg0.75 in the cycloheximide-treated rats. Since the thermoregulatory changes accompanying cold stress are similar to those observed during the genesis of fever, these data suggest that cycloheximide may have a general depressant effect on heat production rather than a specific antipyretic effect.

Effect of acute administration of isoproterenol and angiotensin II, separately and in combination, on water intake and blood pressure of rats.

The effects of administration of isoproterenol, a beta-adrenergic agonist, and angiotensin II, a peptide, separately and in combination, on water intake and blood pressure of rats were studied. The results of 6 factorially designed studies in which 4 different doses of each compound were administered revealed that water intakes increased directly with the logarithm of increasing doses of each. The effect of simultaneous administration of the 2 compounds on water intake was additive at submaximal doses of each. No interactive effects on water intake were observed when the 2 compounds were administered simultaneously in any study. Reduction in urine output appears to be a more sensitive response to administration of isoproterenol than increase in water intake since it was virtually abolished at a dose (2.5 micrograms/kg SC) that had no effect on water intake. The lowest doses of angiotensin II (25 and 50 micrograms/kg SC) had no significant effect on either water intake or urine output. The effect of simultaneous administration of both compounds on urine output was essentially the same as that accompanying administration of isoproterenol alone. Following administration of angiotensin II (150 micrograms/kg, SC) mean systemic blood pressure of unanesthetized, chronically cannulated rats reached maximal levels within 5 min and returned to pretreatment control level by 60 min. Following administration of isoproterenol (25 micrograms/kg, SC), mean systemic blood pressure decreased within 5 min, was maximally depressed by 30 min and had returned halfway to the pretreatment control level by 60 min. Simultaneous administration of isoproterenol and angiotensin II failed to induce a significant change in blood pressure. These results are of particular interest since they show that neither the pressor effect of angiotensin II nor the depressor effect of isoproterenol is essential for the induction of drinking by these 2 compounds.

Reduced responsiveness to beta-adrenergic stimulation in renal hypertensive rats.

Summary Induction of hypertension in rats by encapsulation of both kidneys with latex envelopes was accompanied by a reduced responsiveness to β-adrenergic stimulation. Responsiveness was assessed by measurement of tail skin temperature during 2 hr following s.c. administration of the β-adrenergic agonist, isoproterenol. At doses of 15, 25, 35 and 45 μg isoproterenol/kg of body weight, the responsiveness of tail skin temperature of rats with renal hypertension was significantly less than that of normotensive controls. The mechanism responsible for the reduced responsiveness in renal hypertensive rats remains speculative.